System and method for cell balancing and charging using a serially coupled inductor and capacitor

ABSTRACT

An apparatus for charging a plurality of series connected battery cells, includes a first and second input terminals for providing a charging voltage to the plurality of series connected battery cell. A transformer includes a primary side associated with the charging voltage and a secondary side includes a plurality of portions. Each of the plurality of portions is connected across at least one of the plurality of series connected battery cell. A switch in series between each of the plurality of portions of the secondary side and the at least one of the plurality of series connected battery cells increases an impedance between the portion of the secondary side and the associated one of the plurality of series connected battery cells in a first state and decreases the impedance between the portion of the secondary side and the associated one of the plurality of series connected battery cells in a second state.

PRIORITY CLAIM

The present application is a Continuation of copending U.S. patentapplication Ser. No. 12/650,775, filed Dec. 31, 2009, now U.S. Pat. No.9,397,508; which application claims priority to U.S. Provisional PatentApplication Ser. No. 61/180,618, filed May 22, 2009, and U.S.Provisional Patent Application Ser. No. 61/244,643, filed Sep. 22, 2009;all of the foregoing applications are incorporated herein by referencein their entireties.

RELATED APPLICATION DATA

This application is related to U.S. patent application Ser. No.14/750,702 entitled SYSTEM AND METHOD FOR CELL BALANCING AND CHARGINGUSING A SERIALLY COUPLED INDUCTOR AND CAPACITOR filed on Jun. 25, 2015,and which is incorporated herein by reference in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding, reference is now made to thefollowing description taken in conjunction with the accompanyingDrawings in which:

FIG. 1 is a block diagram illustrating the connection of a cellbalancing circuit with a series connection of battery cells;

FIG. 2 illustrates voltage differences between two cells as a functionof the percent of state of charge of the cells;

FIG. 3 illustrates a schematic diagram of a circuit for charging andbalancing of cells;

FIG. 4 illustrates the battery charging cycle during transition;

FIG. 5 illustrates the battery discharging cycle during transition;

FIG. 6 illustrates an alternative embodiment of FIG. 3;

FIG. 7 illustrates yet another embodiment of the circuit of FIG. 3;

FIG. 8 illustrates yet a further alternative embodiment of the circuitof FIG. 3;

FIG. 9 illustrates a further embodiment of the battery charging andbalancing circuit;

FIG. 10 illustrates a nested configuration of the charging and balancingcircuit;

FIG. 11 is a block diagram of an alternative embodiment of the circuitof FIG. 3 wherein the polarities are reversed on some of the secondarywinding portions; and

FIG. 12 illustrates an alternative embodiment including the plurality ofseries connected transformer portions enabling a stacked configurationthat is scalable.

DETAILED DESCRIPTION

Referring now to the drawings, wherein like reference numbers are usedherein to designate like elements throughout, the various views andembodiments of a system and method for cell balancing and charging areillustrated and described, and other possible embodiments are described.The figures are not necessarily drawn to scale, and in some instancesthe drawings have been exaggerated and/or simplified in places forillustrative purposes only. One of ordinary skill in the art willappreciate the many possible applications and variations based on thefollowing examples of possible embodiments.

Cell balancing and charging systems provide the ability to charge aseries connection of battery cells using a single source. Systems usingmultiple lithium ion or super capacitor cells require balancing of theindividual cells in order to maximize the energy available from thebatteries and to prolong the life of the system. Resistive balancingsystems for charging cells dissipate excess charge as heat are onecommon solution but these types of systems waste energy. Energy transfersystems which are based on a “nearest neighbor” inductive or capacitiveenergy transfer reduce the amount of wasted energy but are complex andgenerally provide less than satisfactory results when transferringcharge over a distance of several cells. Thus, there is a need for acell balancing and charging system that solves the dual problems ofbalancing the state of charge of cells within a stack of battery cellswithout dissipating the energy in an associated resistor and furtherproviding efficient transfer of charge to any cell in the stack withouta distance penalty. The common way of balancing cells within a multicell battery is by discharging the highest cell through a pass elementor alternatively by passing the charge from a pass element to anadjacent cell.

Referring now to the drawings, and more particularly to FIG. 1, there isillustrated a configuration of a cell balancing circuit 102 which isconnected with a series connection of battery cells 104. The chargelevel on a particular battery cell 104 may be moved from one cell toanother in order to balance the charge load across each of the cells104. The cell balancing circuit 102 is responsible for carrying out thiscell balancing/charging functionality. Various types of systems, asdiscussed herein above, exist for transferring the charge from one cellwithin a cell stack to an adjacent cell. However, these systems areoverly complex and expensive and suffer from poor efficiency whentransferring charge over several cells such as from one end of the cellstack to the other.

Referring now to FIG. 2, there is illustrated the voltage differencesbetween two cells as a function of the percent state of charge. Whenbatteries of different impedances or voltages are connected in series,the state of charge of the entire pack is limited. At a low state ofcharge percentage the voltage deviation is very high and can approach500 millivolts deviation. The voltage deviation significantly decreasesand approaches zero as the state of charge approaches 20%. Thus, duringthe charging cycle, the battery including a higher charge voltage mayend up overcharged and damaged, or alternatively, a battery including alower charge level may end up undercharged in order to protect thehigher charge battery. In either case, the battery's cells will notreach their maximum charge voltage. During discharge, the lower chargebattery may pull the total capacity of the series connection to a lowlevel and prevent the taking of maximum charge from the system.

Referring now to FIG. 3, there is illustrated a first embodiment of acircuit for providing charging and load balancing of a series connectionof battery cells 302. The series connection of battery cells 302 areconnected between node 304 and node 306. A charging voltage is suppliedto the battery cells 302 via a voltage source 308 provided between nodes304 and 306. Node 306 comprises the ground node while node 304 comprisesthe input voltage node. A high-side switching transistor 310 MOSFET hasits source/drain path connected between node 304 and node 312. Alow-side switching transistor 314 MOSFET has its drain/source pathconnected between node 312 and the ground node 306.

A resonant tank circuit consisting of inductor 316 and capacitor 320 isconnected between node 312 and node 322. The inductor 316 is connectedbetween node 312 and node 318. The capacitor 320 is connected in serieswith the inductor 316 between node 318 and node 322. A primary side 324of a transformer 325 is connected to node 322 and to the ground node306. The secondary side of the transformer 325 includes a number ofsecondary portions 326, each of which are connected across the terminalsof an associated battery cell 302. The polarity of adjacent secondaryside portions 326 of the transformer are reversed from each other. Aswitching MOSFET 328 has its drain/source path connected between thesecondary portion 326 of the transformer 325 and the negative terminalof the associated battery cell 302. The switch 328 would receive controlsignals from a control circuit (not shown) which also controls switchingtransistors 310 and 314.

During the charging cycle, the system of FIG. 3 is based upon a resonantconverter for every switching cycle, and the amount of energy that isput into the resonant tank by the voltage source 308 is then transferredto the secondary side portions 326. The lowest charged voltage cellswill then take most of the energy transmitted to the secondary side 326from the resonant tank and the highest charged voltage cells the least.Thus, the charge is transferred to the second portion 326 in proportionto the charge on the associated battery cells. In order to add moreprotection and control, the switch 328 is added in series with eachsecondary portion 326 to increase or decrease the overall impedance ofthe battery cell 302. This allows selective charging of the batterycells such as might be required when a cell is to be charged to a highervoltage than other cells. Thus, the cells are balanced during charging.

As can be seen in FIG. 4, the lowest voltage cells are taking all of theenergy provided by the resonant tank while the higher voltage batterycells are sitting idle until the lower battery cells catch up in chargevalue with the higher value tanks. Thus, waveform 402 represents thecharging battery voltage of the lower charge battery cell while waveform404 represents the higher voltage battery.

During the discharge cycle, the input to the primary side 324 of thetransformer 325 will comprise the total series voltages of all of thebattery cells 302. The energy is circulating from all of the batterycells 302 back to the lowest charged cells. FIG. 5 illustrates theampere hour taking every cycle from every cell is the same while theenergy put back into the system is higher for the lower voltagebatteries. Thus, waveform 502 represents the highest voltage batterycell, waveform 504 represents the next highest voltage battery cellwhile waveform 506 represents the lowest voltage battery cell.

The main difference between previous solutions and the implementationdescribed herein above with respect to FIG. 3, is that the energy istaken from the entire stack of battery cells 302 and then redistributedback based on the battery cell that needs more energy than the otherbattery cells. This scheme permits very simple systems whichautomatically distribute charge without the need for a sophisticatedcontrol mechanism. A more sophisticated implementation is possible inwhich balancing may be performed using complex algorithms in a mannerthat maintains optimal performance with a variety of systems over theentire system life. The system may be equally implemented as a charger,balancer or both.

Referring now to FIG. 6, there is illustrated an alternativeimplementation of the circuit of FIG. 3 wherein the MOSFET switches 328between the transformer secondaries 326 and the battery cells 302 arereplaced by diodes 602. In another implementation illustrated in FIG. 7,the switches feeding the tank may be removed and the tank inputgrounded. In this system the switches between the transformersecondaries and the cells are replaced by a suitable arrangement ofswitches and conducting elements. Energy is passed to and from the tankcircuit by selective use of the secondary side switches. E.g. thesecondary side in FIG. 2 becomes both primary and secondary depending onthe configuration of the switch elements. Alternatively, as illustratedin FIG. 8, the lower drive MOSFET 314 may be replaced by a diode 802. Inan alternative control scheme, the currents through the transformerprimary 324 may be sensed to determine a current limit providing an ontime termination point for the circuit and a switch termination timingto determine when to turn off the switching transistors 310 and 314.

Referring now to FIG. 9, there is illustrated a further embodiment ofthe charging/balancing circuit of FIG. 3. The series connection ofbattery cells 902 are connected between node 904 and node 906. Acharging voltage is supplied to the battery cells 902 via a voltagesource 908 provided between nodes 904 and 906. Node 906 comprises theground node while node 904 comprises the input voltage node. A high-sideswitch 910 is connected between node 904 and node 912. A low-side switch914 is connected between node 912 and the ground node 906. A resonanttank circuit consisting of inductor 916 and capacitor 920 is connectedbetween node 912 and node 922. The inductor 916 is connected betweennode 912 and node 918. The capacitor 920 is connected in series with theinductor 916 between node 918 and node 922.

A primary side 924 of a transformer 925 is connected to node 922 and tothe ground node 906. The secondary side of the transformer 925 includesa number of secondary portions 926, each of which are connected acrossthe terminals of the associated battery cell 902. A switch 928 isconnected between the secondary portion 926 of the secondary side 926 ofthe transformer 925 and the negative terminal of the associated batterycell 902. The switch 928 would receive control signals from a controlcircuit (not shown) which also controls switches 915 and 914. Inaddition to the switch 928 connected between the transformer secondaryportion 926 and the battery cell 902, a capacitor 930 is connected inparallel with the switch 928. In this scheme, current may be directed toindividual cells 902 through the selective use of the secondary sideswitches 928 allowing programmable charge balancing or chargeredirection to deliberately produce an unbalanced condition.

Referring now also to FIG. 10, there is illustrated a nested balancingsystem. Nested arrangements are possible in which each of the batterycells are replaced by the balancing circuit 1002 as described previouslywith respect to FIG. 3 and a series of battery cells 1004. The circuitof FIG. 10 comprises a series connection of battery cells 1004 areconnected between node 1005 and node 1006. A charging voltage issupplied to the battery cells 1004 via a voltage source 1008 providedbetween nodes 1005 and 1006. Node 1006 comprises the ground node whilenode 1005 comprises the input voltage node. A high-side switch 1016 isconnected between node 1005 and node 1012. A low-side switch 1014 isconnected between node 1012 and the ground node 1006.

A resonant tank circuit consisting of inductor 1013 and capacitor 1021is connected between node 1012 and node 1022. The inductor 1013 isconnected between node 1012 and node 1018. The capacitor 1021 isconnected in series with the inductor 1013 between node 1020 and node1022. A primary side 1024 of a transformer 1025 is connected to node1022 and to the ground node 1006. The secondary side of the transformer1025 includes a number of secondary portions 1026, each of which areconnected across the terminals of the associated battery cell stack1004. A switch 1028 is connected between the secondary portion 1026 ofthe secondary side 1026 of the transformer 1025 and the negativeterminal of the associated battery cell stack 1004. The switch 1028would receive control signals from a circuit which also controlsswitches 1016 and 1014.

As mentioned previously, rather than a single cell, a series of cells1004 are connected across each of the secondary portions 1026 of thesecondary side of the transformer. Connected across these cells 1004 isthe balancing circuit described previously with respect to FIG. 3. Thus,the battery cells 1004 would comprise the source 308 and the balancingcircuit 1002 would connect with the source at nodes 304 and 306. Thus,each stack of cells 1004 includes its own balancing system 1002 suchthat nested balancing systems may be produced which optimizes thecomplexity/performance trade off.

In an alternative embodiment of the circuit of FIG. 10, the switches1016 and 1014 feeding the resonant tank may be removed and the tankinput grounded. In this implementation, the switches 1028 between thetransformer secondaries 1026 and the cell stacks 1004 are replaced by asuitable arrangement of switches and conducting elements. Energy ispassed to and from the resonant tank circuit by the selective use of thesecondary side switches 1028. Thus, the secondary side becomes both theprimary and secondary depending on the configuration of the switchingelements.

In yet a further embodiment illustrated in FIG. 11, the circuitry isconfigured in substantially the same manner as that described withrespect to FIG. 3. However, the polarities on the secondary sideportions 326 are altered such that some (ideally half) of the secondarywindings have one polarity and the remainder of the secondary windingshave the opposite polarity. The actual sequence between the reversedpolarities within the secondary windings is not important. The benefitthat this configuration provides is that charge may be transferred onboth half cycles of the transformer. The first half cycle feeds thesecondaries with one polarity and the second half cycle feeds those withthe opposite polarity.

Referring now to FIG. 12, there is illustrated a further embodiment thatcomprises a stacked configuration including additional transformer 1233placed in series with the first transformer 1225. The series connectionof battery cells 1202 are connected between node 1204 and node 1206. Acharging voltage is supplied to the battery cells 1202 via a voltagesource 1208 provided between nodes 1204 and 1206. Node 1206 comprisesthe ground node while node 1204 comprises the input voltage node. Ahigh-side switch 1210 is connected between node 1204 and node 1212. Alow-side switch 1214 is connected between node 1212 and the ground node1206. A resonant tank circuit consisting of inductor 1216 and capacitor1220 is connected between node 1212 and node 1222. The inductor 1216 isconnected between node 1212 and node 1218. The capacitor 1220 isconnected in series with the inductor 1216 between node 1218 and node1222.

A primary side 1224 of a first transformer 1225 is connected to node1222 and to the ground node 1206. The secondary side of the transformer1225 includes a number of secondary portions 1226, each of which areconnected across the terminals of the associated battery cell 1202. Aswitch 1228 is connected between the secondary portion of the secondaryside 1226 of the transformer 1225 and the negative terminal of theassociated battery cell 1202. The switch 1228 would receive controlsignals from a control circuit (not shown) which also controls switches1215 and 1214. In addition to the switch 1228 connected between thetransformer secondary portion 1226 and the battery cell 1202, acapacitor 1230 is connected in parallel with the switch 1228. In thisscheme, current may be directed to individual cells 1202 through theselective use of the secondary side switches 1228 allowing programmablecharge balancing or charge redirection to deliberately produce anunbalanced condition.

In the second transformer 1223 of the stacked configuration, a primaryside 1235 of the transformer 1223 is connected in series with theprimary side 1224 of the first transformer 1225. Additionally, a furtherseries of transformer secondaries 1236 are connected across additionalbattery cells 1202 in series with the transformer secondary portion 1226of transformer 1225. As in the first portion of the circuit, a switch1228 would receive control signals from a control circuit (not shown).In addition to the switch 1228 connected between the transformersecondary portion 1236 and the battery cell 1232, a capacitor 1230 isconnected in parallel with the switch 1228. The stacked configuration iscompletely scalable. As many sections as needed may be added in series.Thus, rather than the two illustrated in FIG. 12, any number may befurther added. A single pair of switches 1215 and 1214 and a single tankcircuit consisting of inductor 1216 and capacitor 1220 then feed theseries connected transformer windings.

Thus, the main difference between previous solutions and the presentdisclosure is that the energy is taken from the entire cell stack andredistributed based upon the cells that need more energy than the other.The scheme permits very simple systems which automatically chargewithout the need of a sophisticated control mechanism. Moresophisticated implementations are possible in which the balancing may beperformed using complex algorithms in a manner that maintains theoptimal performance with a variety of systems and over the entire systemlife.

It will be appreciated by those skilled in the art having the benefit ofthis disclosure that this system and method for cell balancing andcharging provides an improved manner of charging/balancing a stack ofbattery cells. It should be understood that the drawings and detaileddescription herein are to be regarded in an illustrative rather than arestrictive manner, and are not intended to be limiting to theparticular forms and examples disclosed. On the contrary, included areany further modifications, changes, rearrangements, substitutions,alternatives, design choices, and embodiments apparent to those ofordinary skill in the art, without departing from the spirit and scopehereof, as defined by the following claims. Thus, it is intended thatthe following claims be interpreted to embrace all such furthermodifications, changes, rearrangements, substitutions, alternatives,design choices, and embodiments.

What is claimed is:
 1. A method, comprising: generating an input currentthrough serially coupled primary windings of multiple transformers,wherein generating the input current includes generating the inputcurrent in response to a resonating of an inductor and a capacitorcoupled in series with the primary windings; and generating, in responseto the input current, charge-balancing currents each flowing through arespective secondary winding of each of the transformers and arespective one of a set of serially connected battery cells such thateach charge-balancing current has a magnitude that is related to avoltage on the respective at least one of the set of battery cells. 2.The method of claim 1 wherein generating the input current includesgenerating the input current in response to a voltage across at leastone of the battery cells.
 3. The method of claim 1 wherein generatingthe input current includes generating the input current in response to acurrent generated by at least one of the battery cells.
 4. The method ofclaim 1 wherein generating the input current includes generating theinput current in response to a current generated by a power supply. 5.The method of claim 1 wherein generating the charge-balancing currentsincludes generating at least one of the charge-balancing currentsflowing in only one direction.
 6. The method of claim 1 whereingenerating the charge-balancing currents includes generating at leastone of the charge-balancing currents only while the input current isflowing in a direction.
 7. An apparatus, comprising: an input circuitconfigured to generate an input current, wherein the input circuitincludes an inductor and a capacitor in series with a first primarywinding and is configured to generate the input current by causing theinductor and capacitor to resonate; and transformers including primarywindings including the first primary winding coupled together in seriesand configured to receive the input current, wherein each of thetransformers includes a respective one of the primary windings, andsecondary windings each configured to generate, in response to the inputcurrent, a respective charge-balancing current flowing through arespective at least one of battery cells that are coupled together inseries such that the respective charge-balancing current has a magnitudethat is related to a voltage across the respective at least one of thebattery cells, wherein the each transformer further includes one or moreof the secondary windings.
 8. The apparatus of claim 7 wherein the inputcircuit is configured to generate the input current in response to avoltage across at least one of the battery cells.
 9. The apparatus ofclaim 7 wherein the input circuit is configured to generate the inputcurrent in response to a current generated by at least one of thebattery cells.
 10. The apparatus of claim 7 wherein the input circuit isconfigured to generate the input current in response to a currentgenerated by a power supply.
 11. The apparatus of claim 7 wherein eachof at least one of the secondary windings is configured to generate arespective charge-balancing current flowing in only one direction. 12.The apparatus of claim 7 wherein each of at least one of the secondarywindings is configured to generate a respective charge-balancing currentonly while the input current is flowing in a direction.
 13. A system,comprising: a ground node; battery cells coupled in series to the groundnode; an input circuit configured to generate an input current, whereinthe input circuit includes an inductor and a capacitor in series with afirst primary winding and is configured to generate the input current byallowing the inductor and capacitor to resonate; and transformersincluding primary windings including the first primary winding coupledtogether in series and configured to receive the input current, whereineach of the transformers includes a respective one of the primarywindings, and secondary windings each configured to generate, inresponse to the input current, a respective charge-balancing currentflowing through a respective at least one of the battery cells such thatthe respective charge-balancing current has a magnitude that is relatedto a voltage across the respective at least one of the battery cells,wherein the each transformer further includes one or more of thesecondary windings.
 14. The system of claim 13, further comprising: apower supply coupled to the ground node and configured to generate apower-supply current; and wherein the input circuit is configured togenerate the input current in response to the power-supply current. 15.An apparatus, comprising: transformers each including: a respectiveprimary winding coupled in series with the primary windings of the othertransformers, and at least one respective secondary winding each havingfirst and second nodes configured to be coupled across at least one of aset of series-coupled battery cells; a circuit configured to generate aninput current through the primary windings; electronic devices eachhaving a first node coupled to one of the first and second nodes of arespective one of the secondary windings and having a second nodeconfigured to be coupled to a respective one of the battery cells; and aseries-resonant circuit in series with the primary windings.
 16. Theapparatus of claim 15, further comprising a ground node coupled to theseries combination of the primary windings.
 17. The apparatus of claim15 wherein the series-resonant circuit includes an inductor in serieswith a capacitor.
 18. The apparatus of claim 15, further comprising: aground node, and; wherein the series-resonant circuit is coupled betweenthe ground node and a series combination of the primary windings. 19.The apparatus of claim 15, further comprising: an input node; a groundnode; wherein the series-coupled battery cells are coupled between theinput node and the ground node; and wherein the circuit includes a firstswitch coupled between the input node and the primary windings; and asecond switch coupled between the ground node and the primary windings.20. The apparatus of claim 15 wherein each electronic device includes arespective switch.
 21. The apparatus of claim 15, further comprisingcapacitors each coupled across a respective one of the electronicdevices.
 22. The apparatus of claim 15 wherein each of the secondarywindings is configured to have a same winding direction.
 23. Theapparatus of claim 15 wherein the secondary windings of each transformereach have approximately a same number turns as the other secondarywindings of the transformer.
 24. A system, comprising: a ground node;battery cells coupled in series to the ground node; transformers eachincluding: a respective primary winding in series with the primarywindings of the other transformers, and at least one respectivesecondary winding each coupled to a respective one of the battery cells;a circuit configured to generate an input current through the primarywindings and including a series-resonant circuit serially coupledbetween the primary windings and the ground node; and electronic deviceseach coupled between a respective one of the secondary windings and arespective one of the battery cells.
 25. A system, comprising: a groundnode; battery cells coupled in series to the ground node; transformerseach including: a respective primary winding in series with the primarywindings of the other transformers, and at least one respectivesecondary winding each coupled to a respective one of the battery cells;a circuit configured to generate an input current through the primarywindings; electronic devices each coupled between a respective one ofthe secondary windings and a respective one of the battery cells; andwherein the circuit includes: an input node; a series-resonant circuithaving a first node coupled to the primary windings and having a secondnode; a first transistor coupled between the input node and the secondnode of the series-resonant circuit; and a second transistor coupledbetween the ground node and the second node of the series-resonantcircuit; and wherein the battery cells are coupled in series between theinput node and the ground node.
 26. The system of claim 25 wherein eachof the secondary windings is configured to have a same windingdirection.
 27. A system, comprising: a ground node; battery cellscoupled in series to the ground node; transformers each including: arespective primary winding in series with the primary windings of theother transformers, and at least one respective secondary winding eachcoupled to a respective one of the battery cells; a circuit configuredto generate an input current through the primary windings; electronicdevices each coupled between a respective one of the secondary windingsand a respective one of the battery cells; wherein the circuit includes:an input node; a series-resonant circuit having a first node coupled tothe primary windings and having a second node; a first transistorcoupled between the input node and the second node of theseries-resonant circuit; and a second transistor coupled between theground node and the second node of the series-resonant circuit; and apower supply coupled between the ground node and the input node.